Systems like San Francisco's never provided freight service. While the track is standard gauged, the track work is designed for the narrow wheels and smaller flanges of the Muni trucks. Standard railroad wheels don't work well at all - because of shallow flangeways on the girder rail and in turnouts the weight is on the flange rather than the wheel, etc. Streetcar systems that also carried standard freight cars used standard railroad wheels or, sometimes, wheels that were wider and had deeper flanges than the San Francisco wheels, but were narrower than standard railroad wheels. (At the Western Railway Museum one can see this by comparing the wheels on unmodified Muni cars, the Key System Streetcars, and interurban cars.)